Field of the Invention
The present invention relates to a control method for a vibration-type actuator, a vibration-type driving apparatus, and an electronic apparatus.
Description of the Related Art
Various types of vibration-type actuators are known which bring a vibrating body and a driven body into pressure contact with each other and excite predetermined driving vibrations in the vibrating body to move the vibrating body and the driven body relatively to each other. FIGS. 12A to 12D are views useful in briefly explaining an outline of an arrangement of a vibration-type actuator 300, which is an exemplary vibration-type actuator of a linearly driving type, and a driving principle of the vibration-type actuator 300. A translational driving apparatus according to an embodiment of the present invention, to be described later, is configured using the vibration-type actuator 300, and hence a description will now be given of the arrangement of the vibration-type actuator 300 and the driving principle thereof.
FIG. 12A is a perspective view schematically showing the arrangement of the vibration-type actuator 300. FIG. 12B is a view useful in explaining electrode patterns formed on a piezoelectric element 304 constituting the vibration-type actuator 300 and their polarizing directions. FIG. 12C is a view useful in explaining a first vibration mode of vibrations excited in a vibrating body 305 constituting the vibration-type actuator 300. FIG. 12D is a view useful in explaining a second vibration mode of vibrations excited in the vibrating body 305.
The vibration-type actuator 300 has the vibrating body 305 and a driven body 301. The vibrating body 305 has an elastic body 303, two projecting portions 302, and the piezoelectric element 304. Here, the vibrating body 305 is fixed to a fixing means, not shown, for the convenience of explanation, and it is assumed that the driven body 301 moves relatively to the vibrating body 305. The projecting portions 302 are formed on one side of the elastic body 303, which has a rectangular flat shape, and integrally with the elastic body 303, or joined to the one side of the elastic body 303 by welding or the like. The piezoelectric element 304 which is an electro-mechanical energy conversion element is joined to the other side of the elastic body 303 which is opposite to the side on which the projecting portions 302 are formed, with an adhesive agent or the like. The vibrating body 305 and the driven body 301 are brought into pressure contact with each other in a projecting direction (Z direction) of the projecting portions 302 by a pressurization means, not shown as a pressurizing direction.
By generating vibrations in a first vibration mode and a second vibration mode in the vibrating body 305 through application of two-phase AC voltages VA and VB to the piezoelectric element 304, the driven body 301 being in pressure contact with the projecting portions 302 is caused to move in a driving direction (X direction) connecting the two projecting portions 302 together. Specifically, in the piezoelectric element 304, two equal electrode areas are formed in the X direction connecting the two projecting portions 302 together, and polarizing directions of the electrode areas are the same (+). In the piezoelectric element 304, the AC voltage VB is applied to a right-side one of the two electrode areas in FIG. 12B, and the AC voltage VA is applied to a left-side one of the two electrode areas in FIG. 12B.
Assuming that the AC voltages VA and VB are of a frequency close to a resonance frequency in the first vibration mode and in the same phase, the entire piezoelectric element 304 expands at one moment and contracts at another moment. As a result, vibrations in the first vibration mode shown in FIG. 12C are excited in the vibrating body 305. Here, the projecting portions 302 are provided close to an anti-node of the vibration in the first vibration mode, and therefore, the projecting portions 302 are vibrated (displaced) in the Z direction. Assuming that the AC voltages VA and VB are of a frequency close to a resonance frequency in the second vibration mode and 180° out of phase with each other, the right-side electrode area of the piezoelectric element 304 contracts and the left-side electrode area of the piezoelectric element 304 expands at the same time at one moment, and this is the other way around at another moment. As a result, vibrations in the second vibration mode are excited in the vibrating body 305. Here, the projecting portions 302 are provided close to a node of the vibration in the second vibration mode, and therefore, the projecting portions 302 are vibrated (displaced) in the X direction.
Thus, by applying the AC voltages close to the respective resonance frequencies in the first vibration mode and the second vibration mode to the electrodes of the piezoelectric element 304, resultant vibrations of the vibrations in the first vibration mode and the second vibration mode are excited in the vibrating body 305. This produces oval motions of the projecting portions 302 within a Z-X plane. The driven body 301 is frictionally driven by the oval motions of the projecting portions 302 and moves in the X direction relatively to the vibrating body 305.
By changing a phase difference between the two-phase AC voltages VB and VA, an amplitude ratio between an amplitude of the first vibration mode and an amplitude of the second vibration mode is changed, and as a result, a speed (moving speed) of the driven body 301 is adjusted. A method for controlling the speed of the driven body 301 by changing the phase difference between the two-phase AC voltages VB and VA is described in Japanese Patent Publication No. 5328259. FIGS. 13A and 13B are diagrams showing a relationship among phase difference, frequency, and speed when the vibration-type actuator 300 is driven. FIG. 13A shows a relationship between control amount and phase difference and frequency. Here, in a region where absolute values of control amounts are small, the phase difference is changed (phase difference control region), and in a region where absolute values of control amounts are large, the frequency is changed (frequency control region). Namely, the phase difference control and the frequency control are switched according to control amounts. In the phase difference control region, the frequency is fixed at an upper limit frequency, and a phase difference is adjusted within a range from an upper limit frequency to a lower limit phase difference (for example, from +120 degrees to −120 degrees) to control reversal of the driving direction, stop, and speed in a low-speed region. In the frequency control region, the frequency is fixed at a lower limit frequency or an upper limit frequency, and frequencies are adjusted within a range from the upper limit frequency to the lower limit frequency (for example, from 98 kHz to 95 kHz) to control speed in a high-speed region.
FIG. 13B shows how the speed of the driven body 301 varies with control amounts. The phase difference control is provided in a low-speed region (−50 mm/s to +50 mm/s), and the frequency control is provided in high-speed regions other than the low-speed region. In the phase-difference control, oval motions produced in the projecting portions 302 are controlled such that oval ratios are changed, and directions of the oval motions are switched by reversing signs of phase differences. In the frequency control, oval amplitudes are controlled such that the oval amplitudes vary with oval ratios of the oval motions being kept constant. On this occasion, a phase difference and a frequency are determined so that a speed of the driven body 301 can be as linear as possible with respect to a control amount. It is known that at this time, characteristics of the vibration-type actuator 300 vary depending on an upper limit frequency setting, and an upper limit frequency is set as described in, for example, Japanese Laid-Open Patent Publication (Kokai) No. H07-95778.
According to the technique described in Japanese Patent Publication No. 5328259 above, the speed is increased until a desired speed is reached by providing control such that the vibration-type actuator is started with an upper limit frequency (hereafter referred to as a “starting frequency”) set at a higher frequency than a resonance frequency, and a driving frequency is lowered. According to the technique described in Japanese Laid-Open Patent Publication (Kokai) No. H07-95778 above, a load on the vibration-type actuator is determined, and a starting frequency is set according to this load. Specifically, when the load is low, a low starting frequency is set so as to quickly start the vibration-type actuator, and when the load is high, a high starting frequency is set so as to prevent a situation in which the vibration-type actuator cannot be started.
However, even in the case where an optimum starting frequency is set according to the load, the control that lowers the driving frequency may cause thrust (torque) to be decreased when the driving frequency is lowered, making the vibration-type actuator inoperable. Moreover, even in the case where the starting frequency is optimized in combination with the phase difference control, the control to lower the driving frequency is provided when the speed lowers due to, for example, an increase in the load during operation. In this case as well, thrust decreases when the driving frequency is lowered, and hence thrust for driving the driven body against the load may not be obtained, resulting in the vibration-type actuator becoming inoperable.